This invention relates to the welding of articles, and, more particularly, to the weld repair of articles made of nickel-base superalloys that are subject to strain-age cracking.
Nickel-base superalloys are extensively used in aircraft gas turbine (jet) engines. These superalloys typically develop their peak strengths through aging to produce a distribution of gamma prime precipitates in a gamma solid solution matrix. Those superalloys which have the highest volume fractions of the precipitates--typically 40 volume percent or more--exhibit the highest strengths and creep resistances at temperatures near their melting points, on the order of 2000.degree. F. Unfortunately, however, these superalloys also have limited ductilities at elevated temperatures, and are consequently subject to cracking due to differential thermal strains in some temperature ranges, a phenomenon termed "strain-age" cracking.
These high-volume-fraction gamma prime superalloys are used in articles such as turbine blades and vanes, which operate at high temperatures for prolonged periods. The superalloys are typically directionally processed to produce blades and vanes with directionally oriented grains or in single-crystal form, which result in the best performance. Notwithstanding the high-temperature capabilities of the alloys, during service the articles are often damaged by hot gas erosion and other types of mechanisms. The tips of the turbine blades and vanes are the regions most prone to such damage. Consequently, these areas are routinely inspected for damage. When damage is found, the article is removed from service and, if the damage is not too severe, repaired.
Repair of the damaged region is accomplished by a welding process. After the damaged area is cleaned, a filler metal is melted and applied to the damaged area. The application is typically accomplished by tungsten inert gas welding, wherein an electric arc is struck between the article and a tungsten electrode, forming a molten pool in the damaged region. A rod of the filler metal is fed into the arc and the pool, melting and filling the damaged region. Upon cooling of the molten filler metal, the resulting weldment is finished as necessary to complete the repair.
While effective to fill the damaged area, the weld repair procedure also creates differential thermal stresses that lead to strain-age cracking in the weldment and in adjacent regions of the welded substrate. This cracking is injurious to the performance of the article, and a number of methods have been proposed to overcome the cracking. In one such approach, the article is pre-heated prior to welding to a temperature greater than its aging temperature, and then maintained at that temperature during the welding operation. The incidence of cracking is somewhat reduced as compared with weld repairing at ambient temperature, but strain-age cracking still remains and is deleterious to the subsequent performance of the repaired article.
The inventors have recognized a need for yet further improvements in the approach used in repairing articles made of superalloys that are susceptible to strain-age cracking. The present invention provides such an improved approach.